Obesity is a condition in which a person has an excess of body fat. It is one of the leading causes of preventable death, contributing to serious health problems such as diabetes, heart disease and cancer. Almost one-third of Americans are obese, according to the National Institutes of Health (NIH).

Weight gain occurs when energy intake exceeds the energy a person expends. When a person takes in an excessive amount of energy, it is stored in the fat cells of adipose tissue. People who are obese have more and larger fat cells than those who maintain healthier body weights.

A combination of high-fat, high-calorie diet (including large quantities of processed foods and refined sugar) and sedentary lifestyle (including lack of regular exercise) is among the main causes of obesity. Genetics also appears to play a role in obesity. If one parent is obese, a child’s chances of being overweight are 40 percent. That percentage doubles if both parents are obese.

Obesity is usually measured by the body mass index (BMI), a calculation based on an individual’s weight and height. BMI is the most common method of tracking weight problems and obesity among adults and is strongly connected to body fat. The criteria for determining obesity are the same for men and women.

People who are obese can usually gain significant health benefits by losing weight. The key is to change the person’s energy balance so that energy output exceeds energy intake. When this occurs the size of the person’s fat cells shrinks, leading to modest but significant weight loss. Even losing as little as 5 percent to 7 percent of total body weight can result in a dramatic decrease in risk for health conditions such as diabetes, cardiovascular disease and stroke.